CPU%20Scheduling

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CPU Scheduling

• Scheduling the processor among all ready
  processes
• The goal is to achieve
• High processor utilization
• High throughput
• number of processes completed per of unit time
• Low response time
• time elapsed from the submission of a request
  until the first response is produced

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Classification of Scheduling Activity

• Long-term which process to admit?
• Medium-term which process to swap in or out?
• Short-term which ready process to execute next?

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Queuing Diagram for Scheduling
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Long-Term Scheduling

• Determines which programs are admitted to the
  system for processing
• Controls the degree of multiprogramming
• Attempts to keep a balanced mix of
  processor-bound and I/O-bound processes
• CPU usage
• System performance

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Medium-Term Scheduling

• Makes swapping decisions based on the current
  degree of multiprogramming
• Controls which remains resident in memory and
  which jobs must be swapped out to reduce degree
  of multiprogramming

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Short-Term Scheduling

• Selects from among ready processes in memory
  which one is to execute next
• The selected process is allocated the CPU
• It is invoked on events that may lead to choose
  another process for execution
• Clock interrupts
• I/O interrupts
• Operating system calls and traps
• Signals

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Characterization of Scheduling Policies

• The selection function determines which ready
  process is selected next for execution
• The decision mode specifies the instants in time
  the selection function is exercised
• Nonpreemptive
• Once a process is in the running state, it will
  continue until it terminates or blocks for an I/O
• Preemptive
• Currently running process may be interrupted and
  moved to the Ready state by the OS
• Prevents one process from monopolizing the
  processor

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Short-Term SchedulerDispatcher

• The dispatcher is the module that gives control
  of the CPU to the process selected by the
  short-term scheduler
• The functions of the dispatcher include
• Switching context
• Switching to user mode
• Jumping to the location in the user program to
  restart execution
• The dispatch latency must be minimal

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The CPU-I/O Cycle

• Processes require alternate use of processor and
  I/O in a repetitive fashion
• Each cycle consist of a CPU burst followed by an
  I/O burst
• A process terminates on a CPU burst
• CPU-bound processes have longer CPU bursts than
  I/O-bound processes

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Short-Tem Scheduling Criteria

• User-oriented criteria
• Response Time Elapsed time between the
  submission of a request and the receipt of a
  response
• Turnaround Time Elapsed time between the
  submission of a process to its completion
• System-oriented criteria
• Processor utilization
• Throughput number of process completed per unit
  time
• fairness

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Scheduling Algorithms

• First-Come, First-Served Scheduling
• Shortest-Job-First Scheduling
• Also referred to asShortest Process Next
• Priority Scheduling
• Round-Robin Scheduling
• Multilevel Queue Scheduling
• Multilevel Feedback Queue Scheduling

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Process Mix Example
Arrival Time
Service Time
Process
1
0
3
2
2
6
3
4
4
4
6
5
5
8
2
Service time total processor time needed in one
(CPU-I/O) cycle Jobs with long service time are
CPU-bound jobs and are referred to as long jobs
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First Come First Served (FCFS)

• Selection function the process that has been
  waiting the longest in the ready queue (hence,
  FCFS)
• Decision mode non-preemptive
• a process runs until it blocks for an I/O

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FCFS drawbacks

• Favors CPU-bound processes
• A CPU-bound process monopolizes the processor
• I/O-bound processes have to wait until completion
  of CPU-bound process
• I/O-bound processes may have to wait even after
  their I/Os are completed (poor device
  utilization)
• Better I/O device utilization could be achieved
  if I/O bound processes had higher priority

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Shortest Job First (Shortest Process Next)

• Selection function the process with the shortest
  expected CPU burst time
• I/O-bound processes will be selected first
• Decision mode non-preemptive
• The required processing time, i.e., the CPU burst
  time, must be estimated for each process

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SJF / SPN Critique

• Possibility of starvation for longer processes
• Lack of preemption is not suitable in a time
  sharing environment
• SJF/SPN implicitly incorporates priorities
• Shortest jobs are given preferences
• CPU bound process have lower priority, but a
  process doing no I/O could still monopolize the
  CPU if it is the first to enter the system

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Is SJF/SPN optimal?

• If the metric is turnaround time (response time),
  is SJF or FCFS better?
• For FCFS, resp_time(39131820)/5 ?
• Note that Rfcfs 3(36)(364). ?
• For SJF, resp_time(39111520)/5 ?
• Note that Rfcfs 3(36)(364). ?
• Which one is smaller? Is this always the case?

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Is SJF/SPN optimal?

• Take each scheduling discipline, they both choose
  the same subset of jobs (first k jobs).
• At some point, each discipline chooses a
  different job (FCFS chooses k1 SJF chooses k2)
• RfcfsnR1(n-1)R2(n-k1)Rk1.(n-k2) Rk2.Rn
• RsjfnR1(n-1)R2(n-k2)Rk2.(n-k1) Rk1.Rn
• Which one is smaller? Rfcfs or Rsjf?

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Priorities

• Implemented by having multiple ready queues to
  represent each level of priority
• Scheduler the process of a higher priority over
  one of lower priority
• Lower-priority may suffer starvation
• To alleviate starvation allow dynamic priorities
• The priority of a process changes based on its
  age or execution history

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Round-Robin

• Selection function same as FCFS
• Decision mode preemptive
• a process is allowed to run until the time slice
  period (quantum, typically from 10 to 100 ms) has
  expired
• a clock interrupt occurs and the running process
  is put on the ready queue

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RR Time Quantum

• Quantum must be substantially larger than the
  time required to handle the clock interrupt and
  dispatching
• Quantum should be larger then the typical
  interaction
• but not much larger, to avoid penalizing I/O
  bound processes

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RR Time Quantum
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Round Robin critique

• Still favors CPU-bound processes
• An I/O bound process uses the CPU for a time less
  than the time quantum before it is blocked
  waiting for an I/O
• A CPU-bound process runs for all its time slice
  and is put back into the ready queue
• May unfairly get in front of blocked processes

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Multilevel Feedback Scheduling

• Preemptive scheduling with dynamic priorities
• N ready to execute queues with decreasing
  priorities
• P(RQ0) gt P(RQ1) gt ... gt P(RQN)
• Dispatcher selects a process for execution from
  RQi only if RQi-1 to RQ0 are empty

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Multilevel Feedback Scheduling

• New process are placed in RQ0
• After the first quantum, they are moved to RQ1
  after the first quantum, and to RQ2 after the
  second quantum, and to RQN after the Nth
  quantum
• I/O-bound processes remain in higher priority
  queues.
• CPU-bound jobs drift downward.
• Hence, long jobs may starve

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Multiple Feedback Queues

• Different RQs may have different quantum values

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Time Quantum for feedback Scheduling

• With a fixed quantum time, the turn around time
  of longer processes can be high
• To alleviate this problem, the time quantum can
  be increased based on the depth of the queue
• Time quantum of RQi 2i-1
• May still cause longer processes to suffer
  starvation.
• Possible fix is to promote a process to higher
  queue after some time

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Algorithm Comparison

• Which one is the best?
• The answer depends on many factors
• the system workload (extremely variable)
• hardware support for the dispatcher
• relative importance of performance criteria
  (response time, CPU utilization, throughput...)
• The evaluation method used (each has its
  limitations...)

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Back to SJF CPU Burst Estimation

• Let Ti be the execution time for the ith
  instance of this process the actual duration of
  the ith CPU burst of this process
• Let Si be the predicted value for the ith CPU
  burst of this process. The simplest choice is
• Sn1 (1/n)(T1Tn)(1/n)?_i1 to n Ti
• This can be more efficiently calculated as
• Sn1 (1/n) Tn ((n-1)/n) Sn
• This estimate, however, results in equal weight
  for each instance

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Estimating the required CPU burst

• Recent instances are more likely to better
  reflect future behavior
• A common technique to factor the above
  observation into the estimate is to use
  exponential averaging
• Sn1 ? Tn (1-?) Sn 0 lt ??lt 1

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CPU burst Estimate Exponential Average

• Recent instances have higher weights, whenever ?
  gt 1/n
• Expanding the estimated value shows that the
  weights of past instances decrease exponentially
• Sn1 ?Tn (1-?)?Tn-1 ...
  (1-?)i?Tn-i
• ... (1-?)nS1
• The predicted value of 1st instance, S1, is
  usually set to 0 to give priority to to new
  processes

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Exponentially Decreasing Coefficients
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Exponentially Decreasing Coefficients

• S1 0 to give high priority to new processes
• Exponential averaging tracks changes in process
  behavior much faster than simple averaging